1. Field of the Invention
The present invention relates to a sawtooth wave generation circuit configured to generate a sawtooth-wave voltage having a sawtooth waveform with a predetermined amplitude and frequency.
2. Description of the Related Art
PWM (Pulse Width Modulation) circuits used in, for example, switching power supplies generate a switching pulse signal by comparing a sawtooth-wave voltage and a control voltage. FIGS. 7A and 7B illustrate a circuit diagram showing a configuration of a conventional sawtooth wave generation circuit and a waveform chart showing temporal changes in a sawtooth-wave voltage generated by the sawtooth wave generation circuit. FIG. 7A is a circuit diagram of the sawtooth wave generation circuit, and FIG. 7B shows an ideal waveform generated by the circuit. As shown in FIG. 7A, the sawtooth wave generation circuit includes: a constant current source 70; a switch 73; an output capacitor 71; and a comparator 72. The output capacitor 71 is charged and discharged when the switch 73 is turned ON and turned OFF, and an output voltage VOUT to be applied to the output capacitor 71 is varied based on comparison with predetermined thresholds VH and VL by the comparator 72. As a result, a sawtooth waveform is outputted. Specifically, when the switch 73 is in an OFF state, the constant current source 70 charges the output capacitor 71 with a constant current. Accordingly, the output voltage VOUT applied to the output capacitor 71 increases, and when the output voltage VOUT reaches the threshold VH which indicates a maximum value, the output of the comparator 72 is inverted and the switch 73 is switched to an ON state. When the switch 73 is in the ON state, the constant current source 70 is connected to the ground, and electric charge stored at the output capacitor 71 is discharged to the ground. As a result, the output voltage VOUT applied to the output capacitor 71 drops rapidly, and when the output voltage VOUT decreases to reach the threshold VL which indicates a minimum value, the output of the comparator 72 is inverted again and the switch 73 is switched to an OFF state. In the conventional sawtooth-wave voltage generation circuit, the series of operations as described above is repeated. As a result, a sawtooth-wave voltage, which linearly increases and suddenly drops repeatedly between the minimum value VL and the maximum value VH as shown in FIG. 7B, is generated.
In such a conventional sawtooth-wave voltage generation circuit, as described above, a sudden drop of the sawtooth-wave voltage occurs due to discharging by the output capacitor 71. There are cases where the operation of the comparator 72 cannot follow such a sudden voltage change, and thereby an operating delay occurs. When an operating delay of the comparator 72 has occurred, the switch 73 is not immediately shifted to an OFF state even if the output voltage VOUT has decreased to reach the minimum value VL. As a result, undershoot occurs, in which the output voltage VOUT falls below the minimum value VL. If undershoot occurs, then a time necessary for the output voltage VOUT to reach the maximum value VH becomes longer due to the undershoot. This lengthens the oscillation period, and it becomes difficult to set the frequency of the sawtooth-wave voltage (i.e., switching frequency) to a high frequency.
As a solution to these problems, there is a known sawtooth wave generation circuit disclosed in, for example, Japanese Laid-Open Patent Application Publication No. 2004-282352.
FIGS. 8A and 8B illustrate a circuit diagram showing a configuration of another conventional sawtooth wave generation circuit and a waveform chart showing temporal changes in respective voltages in the circuit. FIG. 8A is a circuit diagram of the sawtooth wave generation circuit, and FIG. 8B shows ideal waveforms generated by the circuit. As shown in FIG. 8A, the sawtooth wave generation circuit includes: a constant current source 70; two output capacitors 74 and 75 alternately repeating charging and discharging in accordance with results of comparison by a comparison circuit 76 configured to compare a maximum value VH with an output sawtooth-wave voltage VOUT; charging switches 77 and 78 for charging the output capacitors 74 and 75; and discharging switches 79 and 80 for discharging the output capacitors 74 and 75. The sawtooth wave generation circuit is configured such that when the charging switches 77 and 78 are in an ON state, the corresponding discharging switches 79 and 80 are in an OFF state, and such that when the charging switches 77 and 78 are in an OFF state, the corresponding discharging switches 79 and 80 are in an ON state. When the output voltage VOUT has reached the maximum value VH, an output voltage Vcmp from the comparison circuit 76 is inverted, and the ON or OFF state of each of the switches 77 to 80 is switched. For example, if the charging switch 77 is in an ON state and the charging switch 78 is in an OFF state, the output voltage VOUT is equal to a voltage Vc1 of the output capacitor 74. Since the voltage Vc1 of the output capacitor 74 has previously been discharged at the time of the discharging switch 79 being turned ON, the voltage Vc1 of the output capacitor 74 is a minimum value VL. Accordingly, the output voltage VOUT instantly becomes the minimum value VL due to switching by the switches 77 to 80. Moreover, when the charging switch 78 becomes an OFF state, the discharging switch 79 becomes an OFF state, concurrently. As a result, the output capacitor 74 starts to be charged by the constant current source 70. Consequently, the output voltage VOUT increases linearly. While the output capacitor 74 is charged, the discharging switch 80 for discharging the output capacitor 75 is in an ON state. Accordingly, electric charge stored at the output capacitor 75 is discharged, and a voltage Vc2 of the output capacitor 75 rapidly drops to the minimum value VL. However, as shown in FIG. 8B, the voltage Vc1 of the output capacitor 74 and the voltage Vc2 of the output capacitor 75, which increase linearly and drop rapidly between the minimum value VL and the maximum value VH in an alternating manner, are outputted as the output voltage VOUT. Therefore, the sudden voltage drops of the output capacitors 74 and 75 do not affect the output voltage VOUT. Thus, according to such a configuration as disclosed in Japanese Laid-Open Patent Application Publication No. 2004-282352, undershoot of the output voltage VOUT, and increase of the oscillation period due to the undershoot, can be prevented, and a sawtooth-wave voltage with a stable amplitude, the slope of which is steep when the voltage decreases, can be generated.